CLASSIFICATION OF TISSUES. 



125 







Fiff. 25.— Process of Kan'okinesis. A resting stage, chromatin indots. lines ]u8t forming. B. sUffe of 

 ''skein" chromatin in lines, cent ro spheres at opposite poles C. solution of nacJear membrane. £. 

 formation of achromatic spindles, the threads becoming arranged along them, /'.longitudinal ana 

 transverse division of chromosomes completed. J. formation of ''dan^hter nuclei *. M. dan^nter 

 nuclei formed, beginning of separation of the protoplasm. O. P. completion of process by division 

 of protoplasm and formation of middle of lamella. 



transverse division the two halves move 

 to the poles or attraction spheres, and 

 soon a transverse line is seen stretching 

 across the protoplasm of the cell. This 

 soon develops into a new cell wall. The 

 two new cell walls thus formed, each 

 containing its own nucleus, then round 

 out and resemble the parent cell. 



This process, hastily sketched, is con- 

 stantly taking place in all growing ani- 

 mal and vegetable tissues. The details 

 of the process are inexhaustible,(a) 



It is by a continual cell division of this 

 type that the tissues about to be studied 

 have been formed. Originally simple and 

 homogeneous in the embryo, they be- 

 come complex and heterogeneous in the 

 mature plant. 



Classification. 



A satisfactory classification of plant 

 tissues is difficult, as anatomical struct- 

 ure and physiological functions vary so 

 ■widely. The following classifications 



(a) Strassburger Zell und Zell theilung. 

 Gulgnard, 1. c. 



look at plant tissues from four separate 

 standpoints. 



1. Plant tissues may be composed 



of either 



(a) Growing, or meristeniatic, tissue. 



(b) Permanent tissue. 



2. Plant tissues may be classified ac- 

 cording to the shapes of the cells. This 

 is unsatisfactory, but as the names^ of 

 the variously-shaped cells are extensive- 

 ly used, they must be given. 



(a) Equal diameter cells, generally 

 thin-walled. Parenchyma. 



(b) Unequal diameter, generally elon- 

 gated cells, with thickened walls. Pros- 

 enchyma. 



3. A third classification groups plant 

 tissues into systems that are based upon 

 their anatomical position. 



(a) The external epidermal tissues. 



(b) The internal fibro-vascular tissues. 



(c) The ground, or filling, tissues. 



4. A more comprehensive classifica- 

 tion looks at plants from a point of view 

 both anatomical and physiological. 



